Mobile wireless communications systems employing multiple transmit and receive antennas have received much attention lately. This is due—in part—to the fact that the capacity of such systems increases linearly with the minimum of the number of transmit and receive antennas without requiring any additional power or bandwidth. (See for example, G. J. Foschini, “Layered Space-Time Architecture For Wireless Communication in a Fading Environment When Using Multi-Element Antennas”, Bell Labs Tech. J., 1(2):41-59, 1996).
Of the known signal detection schemes employed in contemporary MIMO systems, a maximum-likelihood (ML) scheme is one of the most attractive. Unfortunately however, ML schemes exhibit a computational complexity that is O(MnT), where M is the constellation size and nT is the number transmit antennas. This exponential complexity makes its implementation infeasible or impractical for large systems.
Sphere decoder schemes exhibit performance identical to that of ML detection schemes—oftentimes at a lower average computational complexity. For a system of representative Signal-to-Noise Ratio (SNR) and constellation size however, its computational complexity has also been shown to be exponential in the number of transmit antennas. In addition when one considers that the computational complexity of a sphere decoder scheme is channel dependent and that it typically produces hard decisions only—the attractiveness of a sphere decoder scheme is lessened substantially.
Several suboptimal interference cancellation (IC) based detection schemes have been developed and in particular zero-forcing (ZF) nulling and IC with ordering, and MMSE nulling and IC with ordering are among the best known. (See, for example G. D. Golden, et. al, “Detection Algorithm and Initial Laboratory Results Using V-BLAST Space-Time Communication Architecture”, Elect. Let., 35:14-16, January 1999; and P. W. Wolniansky, et. al., “V-BLAST: An Architecture For Realizing Very High Data Rates Over the Rich-Scattering Wireless Channel”, Proc. 1998 Intl. Symp. Sig. Sys. Elect. (ISSSE'98), pp. 295-300, Pisa, Italy, September 1998) And while these schemes generally exhibit a low complexity, their performance leaves much to be desired as they are generally much inferior to the ML detector schemes.
Recently however, a new class of detection schemes based on sequential Monte Carlo techniques has been proposed which approach the performance of ML schemes while exhibiting a computational complexity that is linear in M and nT. In addition, these SMC-based schemes are both “soft-input” and “soft-output”, and are particularly well suited for turbo processing in coded MIMO systems. As the constellation size or the number of transmit antennas increase however, the complexity of existing SMC MIMO detectors—while considerably lower than ML detectors—becomes unacceptably high.